Do you want to have a recommendation for a shield ?Depending on what you want to achieve with you r project, getting a micro controller board that supports USB host mode natively may be e best option.

The Teensy 2 and Teensy++2 support it natively, are much smaller then an UNO and less expensive.http://www.pjrc.com/teensy/index.html. They are fully Arduino compatible and come with a large number of examples, including for USB host mode.